I believe that rust now defaults to the system's malloc implementation instead of bundling jemalloc, so that bit is no longer necessary:

    #![feature(alloc_system)]
    extern crate alloc_system;

As for the part that removes libstd it's how you'd generally do it for a bare metal program (i.e. bootloader/OS) since you can't really use it without an OS unless you manually reimplement all sorts of primitives (memory allocations being the most obvious one).

Now if you do use libstd it's true that it's significantly larger that libc, but it's also vastly more powerful. You don't have anything like String, Vec or iterators in libc for instance, just a bunch of relatively low-level functions and wrappers around syscalls. It's also a fixed cost, so obviously for a simple Hello World there's a massive overhead but for a more complex application it should be less noticeable.

I mean when you think about it even the C Hello World is ridiculously bloated. If you were to write the equivalent program in assembly without any dependency on the libc you could probably get a binary that would be significantly smaller (and most of its size would really be the ELF metadata).